Polarimeters and ellipsometers are comprised of optical elements such as polarizer and retarder systems. Polarimeter systems allow the polarization state of a polarized beam of electromagnetic radiation to be determined, and ellipsometer systems allow detection of change in polarization state of a polarized beam of electromagnetic radiation resulting from interaction with a sample system to be determined, said change in polarization state being associated with optical and physical properties of said sample system. For general information it is noted that the polarization state of a polarized beam of electromagnetic radiation is determined by:
a. ratio of orthogonal components, (related to PSI); PA1 b. phase angle between said orthogonal components, (related to DELTA); PA1 c. absolute value of one orthogonal component; and PA1 d. the direction of rotation, or handedness. PA1 a source of electromagnetic radiation; PA1 a polarizer; PA1 a compensator; PA1 an analyzer; and PA1 a detector system;
Continuing, in the Ultraviolet-Visible-Near Infrared spectral region, (ie. wavelengths between one-hundred-ninety (190) and two-thousand (2000) nanometers (nm)) polarizer elements which exhibit nearly ideal characteristics are readily available. However, no polarizer systems are available which provide ideal characteristics in the infrared range of wavelengths, (ie. wavelengths in the range of approximately two (2) to fifty (50) microns). There is thus identified a need.
As well very few retarders are available which provide even a remotely achromatic response over any wavelength range of from in the infrared to say, eight hundred (800) nm and above. Again, a need is thus identified.
An ideal polarizer would pass only linearly polarized electromagnetic radiation aligned with the fast axis thereof, and would reject all electromagnetic radiation in an orthogonal orientation. That is, the extinction ratio would be essentially infinite. The Mueller Matrix for an ideal polarizer is provided below: ##EQU1##
An ideal Retarder system should enter a phase retardation between orthogonal components of polarized electromagnetic radiation without preferentially modifying the intensity of either orthogonal component thereof. The Mueller Matrix of an ideal Retarder is: ##EQU2## where "r" is the entered retardence.
As even very good Retarder systems tend to preferentially modify one orthogonal component of an electromagnetic beam of radiation, (including those presented in this Disclosure), it is necessary to modify said Mueller Matrix to account for said effect. The Mueller Matrix of a Retarder system which accounts for preferential modification of one orthogonal component of a polarized beam of electromagnetic radiation is: ##EQU3## where "r" is again the retardence entered. Note that where Retarder system PSI (.psi.) is forty-five (45) degrees, said Mueller Matrix reduces to the ideal Mueller Matrix.
It is additionally noted that the value of "r" should be in a range where an ellipsometer system in which it is a component is not severely sensitive to changes therein as, for instance, a function of wavelength. In Rotating Compensator Ellipsometers, it is disclosed that a value of "r" between ninety (90) and one-hundred-fifty (150) degrees is generally acceptable. It is also noted that typical off-the-shelf Retarder systems often exhibit an "r" with a (1/wavelength) response such that "r" values are not within said 90 to 150 degree range, when observed over a wavelength range of say, two-hundred-fifty (250) to one-thousand (1000) nm.
It is a requirement of an ideal optical element that a beam of electromagnetic radiation caused to interact therewith not have its direction of propagation deviated or displaced thereby. This is especially critical where an optical element must be rotated in use.
It is further desirable that an optical element not exhibit sensitivity of, for instance, extinction ratio, or retardence entered between orthogonal components of an electromagnetic beam of radiation caused to interact therewith, as a function of beam alignment with respect thereto.
As well, it is desirable that optical elements be easy to fabricate and that fabrication be from easily obtainable materials.
The practice of ellipsometry requires that data reflecting change in polarization state of an electromagnetic beam of radiation resulting from interaction with a sample system be obtained and that said data be compared to data generated by use of a proposed mathematical model. Said mathematical model must take into account all nonidealities of optical elements present in the ellipsometer utilized. It is thus preferable to have as few nonidealities present in optical elements as is possible, in order to simplify mathematical model complexity.
With an eye to the present invention, a Search of Patents was conducted. Said Search was focused on polarizers suitable for use in the infrared, and on compensators which might provide relatively stable retardation over a range of wavelengths without imposing deviation or displacement in a beam of electromagnetic radiation caused to pass therethrough.
Regarding compensators Patents were found which show elements with geometry somehow similar to geometry of present invention compensators, but the present invention use was not found. In particular attention is directed to the Figure in Pat. No. 548,495 to Abbe; FIG. 2 in Pat. No. 4,556,292 to Mathyssek et al.; FIGS. 1 & 4 in Pat. No. 5,475,525 Tournois et al.; and FIG. 10 in Pat. No. 5,016,980 Waldron. Pat. No. 3,817,624 to Martin and Pat. No. 2,447,828 to West were also identified.
Regarding polarizers, Patents were also identified. For Instance, Pat. No. 4,221,464 to Pedinoff et al. shows a Double Plate Brewster Angle Polarizer (10) (12) which serves to avoid back reflections, which is combined with a Wire Grid Polarizer (28) present on Plate (14). A purpose of the invention is to increase the extinction coefficient of the Wire Grid Polarizer, by combining it with the Double Plate Brewster Angle Polarizer (10) (12).
A Pat. No. 5,177,635 to Keilmann, shows an Infrared Polarizer Structure of patterned metal strips on a transparent material, but does not suggest a dual polarizer arrangement.
While a Pat. No. 4,961,634 to Chipman et al., shows two polarizers made of CdS and CdSe respectively, in series, there is no angled arrangement therebetween suggested.
A Pat. No. 452 to Mertz, shows multiple Wire Grids oriented at angles with respect to one another. The system is an Interferometer in which said one said Wire Grid is rotated with respect to another.
A Pat. No. 3,439,968 to Hansen, shows an Infrared Brewster Angle Polarizer.
A Pat. No. 3,428,388 to Kuebler et al., shows a UV "Biotite" based Brewster Angle Polarizer.
A Pat. No. 5,187,611 to White et al., shows a system which illuminates an object while avoiding reflection and glare.
Pat Nos. 4,733,926 to Title; 5,548,427 to May and 5,402,260 to Tsuneda et al. were also identified.
It is specifically noted that a Combination system of a dual wire grid polarizer in combination with compensators), (particularly In the context of an IR Ellipsometer/Polarimeter system), was not found.
Optical elements providing Polarizer and Retarder system characteristics which approach ideal over even some limited spectral range, it should be appreciated, would provide utility. The present invention provides advancement toward the goal of achieving ideal optical Polarizer and Retarder system elements which demonstrate acceptably ideal behavior over relatively large wavelength ranges.